The present invention relates to a key telephone system in which a plurality of key telephones and a key service unit are coupled by three transmission cables.
A key telephone system has been known in which a voice signal and control signals are transferred through three transmission cables between each of key telephones and a key service unit.
To make a telephone call through this kind of key telephone system, a subscriber picks up the handset of a key telephone, pushes an outside line selection key, and then pushes proper dial buttons. An up-control information signal generated in response to the selection key operation is transmitted to the key service unit through a first transmission cable. Upon receipt of the control signal, the key service unit energizes the crosspoint corresponding to the outside line selected in a crosspoint module to couple the key telephone with the selected outside line through a second transmission cable. In response to an incoming call signal transmitted from a telephone exchange, the key service unit transmits a down-control information signal to the key telephone through a third transmission cable to light the corresponding lamp of the key telephone for indicating the outside line. Then, if a called subscriber picks up the handset and presses the outside line selection key corresponding to the lighted indication lamp, the key telephone and the outside line through which the incoming call signal is transmitted, are connected to complete a message, or communication channel.
As described above, in the conventional key telephone system, the three transmission cables each formed of a pair of wires are used to transmit the voice signal for message, and up- and down-control information signals therethrough. The power is supplied by using the first and third transmission cables for up- and down-control information signals.
In such a conventional key telephone system, it is impossible to trasmit an additional information signal between the key telephone and the key service unit. For example, it impossible to transmit an additional information signal between the key telephone and the key service unit. For example, it is impossible to execute a voice paging for the key telephone being now busy. That is, only one pair of transmission wires, i.e. the second transmission cable is allotted as a message, or communication channel, so that, when used for the message transmission, the second transmission cable can not be used for the voice paging. In the prior art, the second transmission cable is also alloted for the voice paging so that the message transmission is adversely effected. A similar problem is found in the time-division multiplex system when another control information signal is transmitted at a speed different from that of the up- and down-control information signals.